Angle-Torque: Fact or Fiction?
TIM BARBO, Knoxville, TN: I will be assembling an '83 5.0 H.O. engine in the near future, built to factory specs with stock heads. My question is about the head bolt torque specs. My repair manual and gasket set say the head bolts should be torqued in three steps, first at 25-35 lb-ft, second at 45-55 lb-ft, third an additional 85 to 95 degrees. What is the reasoning behind the additional 85-95 degrees?
Is there a torque value I can use safely for the third step? This will be my third engine build. I used a final torque number for the head bolts on the other two. These engines were both from a '70s car. I understand I can go the additional 85-95 degrees and be safe, but I'm curious as to why.
Jeff Smith: The head-bolt tightening sequence you're referring to is a torque-angle method and the bolts are referred to as torque-to-yield fasteners. Many late-model engines such as the Gen III and Gen IV GM engines use this method. Of course, this requires the use of special head bolts designed to stretch more than a standard bolt. This establishes a more even torque load across the cylinder head. Generally, these are also one-time-use fasteners. However, according to our reference books, Ford was still using standard non-torque-to-yield head bolts in your '83 Ford 5.0 H.O. engine, converting to torque-to-yield bolts later in the '90s. A torque-to-yield head bolt can be identified by its integrated washer.
All of this is done to improve sealing for the head gasket, since the small-block Ford uses only four bolts per cylinder to seal combustion pressure. If you know your head bolts are the traditional style, we recommend a three-step torque process of 25 lb-ft, 45 lb-ft, and 72 lb-ft in a circular pattern working from the center of the head outward. However, to improve your chances of better cylinder sealing, we'd suggest using a set of ARP head bolts (PN 154-3601, $37.88 from summitracing.com). Their 71/416-inch threads are rolled rather than cut and offer far superior engagement with the block compared to stock head bolts. This prevents pulled threads, especially with a well-seasoned block like yours. We'd also recommend carefully cleaning the threads in the block, but only run a tap through the bolt holes once. This way you don't remove more material than necessary. Further cleaning can be accomplished with a small brush. Blow out all chips from blind holes with compressed air and use either oil or an assembly lube on the threads and between the bolt head and the washer. For the bottom row of head bolt holes tapped into the water jacket, you must use a thread sealer to prevent coolant leakage past the threads.
Terry McGean: I suspect that the torque spec you found was intended for the last of Ford's 5.0 engines, the ones used mostly in Explorers. The last few years of production used torque-to-yield head bolts, and as Jeff pointed out, these are a one-time use deal. Your original '83 bolts could be reused, but since you probably don't know their history-like how many times they've been torqued, and how accurately-you should probably follow Jeff's suggestion and get a set of new bolts. The concern over a used head bolt is that someone before you may have over-torqued it, damaging the bolt in the process and leaving it likely to break or provide insufficient clamping. I agree that the bolt holes should be cleaned, but I'd steer you toward one of ARP's thread chasers rather than a tap. These tools are made specifically for cleaning threads and are less aggressive than a tap, which is really intended for cutting new threads. Using a tap will remove a little material, and we've seen the threads pull out of engine block decks on seasoned motors that were treated to a cleaning with a tap. It probably won't happen the first time you torque down your heads, but it might the next.
More Info
Automotive Racing Products (ARP); Ventura, CA; 805/339-2200; arp-bolts.com